You should be able to move while casting and fishing, for fidgety players. Could be a mechanic that some fish like the line to move in certain ways.

Swapping items to the active slot is VERRRY tedious. Minecraft/Terraria did this right.

The demo should get right into the gameplay mechanics. While your eventual players will sit through a slow introduction, the demo should have all of the mechanics unlocked. The full game could spend some time introducing mechanics though.

Seconding the zeroquest's issues with the shop. Another solution I've seen is an 'undo', that is buying back the item you just sold for the same price.

Overall very comfy and no outstanding performance problems like I've seen in other demos.